GENERALIZED ION RING INSTABILITY DRIVEN BY IONS WITH A PARTIAL SHELL DISTRIBUTION

A generalized ion ring instability driven by energetic ions with a she ll velocity distribution, which has a central pitch angle at 90 degree s, is studied. We consider waves propagating in both parallel and obli que direction as well as different pitch angle spreads of a partial sh ell. These unperturbed distributions represent different stages during the time evolution of an ion distribution, that is a ring initially. The evolution is attributed to pitch angle scattering due to wave-part icle interactions. Numerical results obtained by us show that the inst ability diminishes rapidly, especially for oblique propagation, while the pitch angle spread increase.